The center of gravity of an unladen aircraft may shift once the aircraft is loaded with cargo and/or passengers. For a cargo or military aircraft, the center of gravity (forward/aft, port/starboard) may be calculated by a load master who may use a calculator device and/or computer program based on various assumptions. A load master may weigh pallets, vehicles, or other transportable items, then calculate a center of gravity based on the weights and planned location of these transportable items. A load plan is typically tolerant of some variation, but an unexpected or extreme variation could lead to a hazardous condition for the aircraft. For example, the load pallets may be queued up in order and loaded quickly. However, it is possible the cargo pallets may be loaded in the wrong order, or may be loaded into a wrong location or secured improperly. For a commercial airliner, an assumption for load can be made based on the average weight of a passenger and the expected seat assignments and the location and average weight of a luggage/cargo carrier.
Also, aircraft typically have several fuel tanks, and these tanks have gauges to indicate the amount (number of pounds) of fuel in each tank, thereby providing load information. However, it is possible the gauges could be faulty, or some other confusion in fueling the aircraft (e.g. there is confusion as to which tank fuel was added or fuel is added to an incorrect tank causing an unbalanced condition), which could lead to an imbalance that might not be noticed by the flight crew. Some aircraft include weight sensors in the landing gear and can directly measure the weight-on-wheels. Other aircraft have no such weight sensors, and therefore rely entirely on the skill of the ground crew and load master to ensure proper balance. For a land vehicle, such as a car or a light truck, various sensors of critical systems like tire pressure gauges within the tires report the pressure and some vehicles have automatic load leveling to ensure a smooth ride. However, these systems tend to measure only static quantities and may adjust only periodically. Such measurements may not be sufficient for vehicle ground travel such as aircraft taxi conditions. A drawback with such systems is that the actual weight is never measured.
For an aircraft taxing to takeoff, the last chance to abort a takeoff in a mis-loaded or shift-loaded aircraft condition is right after the initiation of rotation during the takeoff run. As the aircraft is rotating and before liftoff, the aircraft has a new posture and dynamic behavior. If a cargo palette or luggage module is not anchored properly and shifts backwards during rotation, the aircraft can suddenly experience a shift in the center of gravity and the nose can unexpectedly positively pitch up at a greater rate than intended. A pilot may not feel this in their hands on the yoke or steering device due to feedback insensitivities or distractions such as increased noise levels, looking for possible ground cross-traffic or potential bird strikes. The shifting of an improperly tethered load or breaking of a tether could happen almost immediately after rotation begins, so there can enough time to abort the takeoff prior to achieving liftoff speed or running out of runway if the mis-load or shift-load aircraft condition is immediately detected.
In this manner, the pitch behavior and forward-aft displacement of the center of gravity due to mis-loading or shift-load can be more important than the side-to-side mis-loading or shift-load. The lever-arm or moment effect of a mis-load or of a shift-load forward-aft can provide a larger effect because the possible displacement distances can be longer due to the elongated nature of many aircraft where the cargo/luggage loadable sections of the fuselage are much longer than they are wide. There are many potential causes of load imbalance, including:
mis-loading of cargo (not loading pallets according to plan, wrong order), improperly securing of loads where a tether or anchor could break or a floor anchor becomes unlocked resulting in a shift-load condition, mis-loading of fuel in various wing or other tanks, leakage of fuel from a tank and/or degradation of an undercarriage element (e.g. a gear or strut).
Many carrier aircraft have an undercarriage with main gear under each wing or attached to the fuselage under the wing attachments and a nose gear assembly arranged at three-points of a triangle. This landing gear configuration, often referred to as, “tricycle gear”, provides good protection against tipping forward during landing since the nose wheel is raised higher than the main gear during the landing flare, and is also far forward of the center of gravity at the nose gear touch down. The main landing gear of the tricycle arrangement enables side-to-side stability against wind gusts or irregular landing where a bounce on one main gear may impart a roll motion to the aircraft. Tricycle gear also provides more optimal take off maneuvering by allowing rotation prior to liftoff from the runway, which increases the angle of attack and thus the lift of the wings.
A system and method is needed that can detect during taxi or at rotation the occurrence of such a mis-load or shift-load condition as described.